DYNAMICAL GEOLOGY. 249 



fathoms in depth. He accepts also the theory of a volcanic 

 basis as the best explanation of atolls. The accuracy and 

 completeness of Ehrenberg's researches in the Red Sea have 

 been since confirmed by some of the best German authorities 

 on coral life Haeckel, Klunzinger, Walther. 



The reefs of the Bermuda Islands were described by Nelson 

 in 1837, and this author demonstrates reef-growth upon a rock- 

 basis neither volcanic nor even firm and compact; in his 

 conclusions regarding the origin of atolls he supports Ains- 

 worth's views. 



One of the most attractive books of the nineteenth century 

 was undoubtedly Charles Darwin's great work, The Structure 

 and Distribution of Coral Reefs, published in 1842. Ehren- 

 berg's work had paved the way for broader conceptions about 

 coral reefs; in it the barrier reef, which had in the older litera- 

 ture been kept in the background by the more aggressive 

 features of the atoll, for the first time received its meed of 

 attention. The balance of scientific knowledge regarding the 

 barrier and the atoll was now fairly equal, and Charles Lyell's 

 indication of possible modifications that might ensue in the 

 reef-form under the influence of differential crust-movements 

 also lay open in the recent literature when Darwin's master- 

 mind came to the formidable task of considering all the known 

 data and constructing a scientific generalisation. 



Charles Darwin, while a member of the Beagle Expedition 

 between 1832 and 1834, examined a large number of coral 

 reefs, atolls, and volcanic islands in the Pacific Ocean, and 

 described them with remarkable method and clearness. He 

 classified coral structures in three groups, now universally 

 accepted atolls or lagoon reefs, barrier reefs, and fringing 

 reefs. This special work contains a map of the geographical 

 distribution of the coral reefs, and enriches our knowledge by 

 a wealth* of new observations on the mode of life of the corals, 

 as well as on the relative part taken by the various coral types 

 in the construction of the reefs. 



Darwin confirmed the fact that reef-corals only live at small 

 depths and in tropical areas, and proposed upon the basis of 

 crust subsidence an ingenious theory of reef-growth which 

 connected the three chief varieties of reefs by intermediate 

 stages. Darwin's theory assumes that every atoll reef was 

 originally the fringing reef of some island, but owing to the 

 subsidence of the ocean-floor, the fringing reef was gradually 



